1. Technical Field
The present invention relates to an image display device.
2. Related Art
In the past, projectors have been known as image display devices capable of displaying large-screen images. As light sources for the projectors, laser sources are thought to be hopeful from the viewpoint that high contrast ratio, excellent color reproducibility, quick lighting, downsizing, longer life, and so on become achievable. Use of the laser source makes it possible to obtain a number of advantages described above on the one hand, but it also makes it easy for the speckle noise due to the coherent property of laser beams to occur on the other hand. If the speckle noise is viewed, the sensation of glare might be provided to the observer of the displayed image to thereby degrade the image quality in some cases.
As one of the methods capable of reducing the speckle noise, there can be cited a method of superimposing high-frequency noise on the speckle noise, for example, a method of widening the diffusion angle of the image light (reduction in F-number). In particular, it is preferable to widen the diffusion angle of the image light so that the light intensity in the Fourier transform plane of the projection optical system has a ring-like distribution having a peak around the light axis. By making the light intensity have the ring-like distribution, it becomes possible to increase the light contributing to the reduction of the speckle noise, and at the same time, to reduce the light covered by the projection optical system compared to the case of forming a broad distribution around the light axis. Thus, it becomes possible to effectively reduce the speckle noise, and at the same time, degradation of the light efficiency can also be prevented.
In order for widening the diffusion angle of the image light, it is sufficient to dispose the diffusion section at the position where the image light is focused into an image (see, e.g., JP-A-2009-42372 (Document 1)). In Document 1, a diffusion plate formed of a diffraction optical element such as a computer generated hologram (CGH) is disposed at the position where an intermediate image is formed in the projection optical system, and thus, the diffusion angle of the image light is widened by the diffusion plate.
According to the technology of Document 1, the speckle noise can effectively be reduced, but there arises a problem of growing in size of the projection optical system. As a method of widening the diffusion angle of the image light at a position other than the position where the intermediate image is formed, there can be cited a method of disposing the diffusion section in the vicinity of the image formation surface (e.g., a liquid crystal layer) in a light modulation element (e.g., a liquid crystal light valve).
According to this method, although the growing in size of the device can be avoided, the following problems might be caused. In order for assuring the good light efficiency in this method, it is required to incorporate the diffusion section capable of controlling the light intensity to have a desired distribution such as a CGH into the light modulation element. In order for making the pattern of the CGH correspond to the pixels, microfabrication becomes necessary, and therefore, the device cost of the light modulation element might rise, or the device configuration might be complicated. Further, if microlenses are disposed on the light entrance side of the light modulation element for the purpose of guiding the source light to the pixels, the diffusion section might fail to function accurately due to the disturbance of the wavefront caused by the microlenses. If the diffusion section becomes to fail to function accurately, it becomes unachievable to make the light intensity distribution of the image light have a desired pattern, and therefore, the light efficiency is degraded.